In U.S. Pat. No. 4,091,683 issued May 10, 1978, a system for downhole measurement of pressure is disclosed in which an interrelated capacitance is utilized for measuring pressure. In U.S. Pat. No. 4,322,775 issued Mar. 30, 1982, a sensor construction is disclosed in which the measurements of two capacitors in a sensor construction are obtained and in which there is compensation for the effects of gravity and temperature and in U.S. Pat. No. 4,624,136 issued on Nov. 25, 1986, a typical downhole oil field tool for pressure and temperature is disclosed is illustrated.
While the present invention has a wider application, one of the critical areas of concern to the evaluation of oil field reservoirs involves the determination of downhole pressure and temperature. For this reason, pressure and temperature measurements are taken during the actual drilling operations in procedures called "drill stem tests". Also, after a well is completed, it is a common occurrence to measure the downhole pressure and temperature over a period of time for "production tests". In oil field applications, therefore, measurement of pressure and temperature is an important factor and it is made even more important in that the accuracy of the measurement can be a significant factor in determining the extent of oil reserves available from underground production. The extent of oil reserves is an asset and thus precise measurements are necessary for accurate asset evaluations.
Oil field practices being what they are, cause equipment to be subjected to rough treatment which translates into shock loading for instrumentation in a well tool. It is not uncommon for instrumentation to be dropped at the earth's surface in a shop, or at the well site, or transported at a haphazard speed into or out of the well bore. In some operations, it is sometimes possible to have the pressure and temperature sensing device connected up to the system when a shape charge perforator or other explosive device is detonated in the well bore. Thus, the pressure and temperature measuring equipment is subject to moderate to severe shock loads at any time and preferably a construction which minimizes the effects of shock loading is highly desirable.
Another significant factor in pressure and temperature measurements is the fact that a temperature change can occur in a downhole well situation and because the tool has a temperature response lag factor, the pressure recordings can be inaccurate until the instrument or tool stabilizes at the changed temperature. Thus, there can be a time span of inaccuracy in the pressure measurements while a temperature stabilization process occurs.
In a typical oil field production test of temperature and pressure, the instrumentation is lowered typically on an electrical wireline, slick line or tubing into the well bore and the tool senses the pressure and temperature in the well bore over a period of time. This data collected over a period of time is either read out at the surface or retained in a downhole memory for subsequent analysis.
In a non-oil field application, the transducer can be utilized for measuring force, i.e., weight or load directly with a high degree of accuracy.